The invention relates to a method of heating, holding or heat treatment of metal material in an atmosphere of processing gas consisting predominantly of nitrogen and produced at, or in close vicinity to the location where the heating, holding or heat treatment takes place.
In the metallurgical industry many heat treatment processes are carried out in an atmosphere of processing gas consisting to a great part, often 90 percent or more, of nitrogen gas. The processing gas sometimes is a pure protective gas serving to prevent an undesired influence on the material, such as surface oxidation, which would occur but for the protective gas. In other cases, the processing gas should play an active part in the process, e.g. bring about carburization or decarburization of the material. The above is also true for other processes which are not, strictly speaking, heat treatment processes but only serve to increase the temperature or to maintain the temperature constant, that is, serve for heating and holding, respectively. However, the term "heat treatment" is hereinafter used as a collective term embracing all the aforesaid processes.
Often the heat treatment takes place in very large furnaces which may be of a multiplicity of different types and may operate batchwise or continuously. Regardless of the type of furnace, equipment is provided to ensure that the desired atmoshpere is maintained in the furnace chamber during the heat treatment, that is, to ensure that the atmosphere is made up only of the processing gas, which is controlled at least in respect of its composition.
The processing gas is produced on the site, that is, within the plant or workshop where the heat treatment takes place. Usually the equipment for the production of the processing gas is located in very close vicinity to the furnace.
Very large quantities of the processing gas are required; a processing gas consumption of 500-1000 m.sup.3 /h at a single plant is not uncommon. The principal raw material for the production of the processing gas is hydrocarbon gas (liquefied petroleum gas, natural gas and the like), which is thus consumed in considerable quantities; a processing gas consumption of 700 m.sup.3 /h corresponds to an annual consumption of liquefied petroleum gas of 500 metric tons. The raw material cost for the processing gas therefore is substantial.